1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor for a magnetic read head, more specifically to the formation of an abutted junction between the sensor element an conducting lead layers.
2. Description of the Related Art
Magnetic read sensors that utilize the giant magnetoresistive (GMR) effect for their operation are generally formed with abutted junctions. These junctions are the interfacial regions wherein the GMR sensor element is contacted electrically by a current lead layer and possibly also contacted magnetically by a longitudinal magnetic bias layer. The current lead layer injects the current into the sensor element that enables its magnetic moment variations and concurrent resistance variations to be converted into measurable voltage variations, while the magnetic bias layer maintains the integrity and stability of magnetic domains within the magnetic layers of the element. The method of forming the abutted junction is critical to the performance of the read element since the track width of the read portion of the element (ie., the width of its active portion) is essentially defined by the distance between the two junctions. If the trackwidth of the read element is to be held below about 0.5 microns, then the prior art methods of forming the abutted junctions have notable shortcomings.
In the prior art, as depicted schematically in FIGS. 1, 2, 3 and 4, abutted junctions are formed by removing portions of a deposited GMR layer with an ion-milling process using a bi-layer lift-off stencil as an ion-milling mask. FIG. 1 shows a substrate (1), over which has been formed a GMR sensor layer (2), which is to be milled to an appropriate shape for forming abutted junctions. In FIG. 2, an ion-milling mask has been formed by depositing a layer of photoresistive material (6) over a layer of PMGI (polydimethylglutarimide) polymer (4). By making use of the differential solubility of the photoresist polymer layer relative to the PMGI polymer layer, the PMGI layer can be undercut (8) beneath the photoresist. This structure is called a bi-layer lift-off stencil or lift-off mask. When the ion-beam etch (IBE) is then applied to the masked region (FIG. 3), the unnecessary portions of the GMR layer are removed (10), forming the required abutted junction profile (12) of the remaining GMR material. Having done its job as an ion-milling mask, the lift-off stencil now (FIG. 4) serves as a stencil for the deposition of a conductor lead layer and a magnetic bias layer (14).
Chen et al. (U.S. Pat. No. 5,664,316) discloses a multilayered conductive lead structure consisting of layers of conductive material alternating with layers of refractory metal, such as layers of gold/nickel alloy alternating with layers of tantalum. This structure is brought into contact with a magnetoresistive (MR) layer at an abutted junction. The junction is formed by a subtractive process such as ion-milling wherein a bi-layer resist formed by a relatively thin underlayer and a thick imaging layer is used as a stencil to define each edge of the MR layer. The undercut is formed by dissolution of the underlayer in a suitable developing medium.
Chen et al. (U.S. Pat. No. 5,491,600) discloses the formation of an MR sensor in which a PMGI layer formed between a photoresist layer and a capping layer in order to facilitate the lift off process. The photoresist masks the active region of the sensor during the etching and deposition process for the conductive lead structures.
Pinarbasi (U.S. Pat. No. 5,883,764) discloses a method for forming abutted junctions, at which very thin and highly conductive lead layers, deposited over longitudinal bias layers, are connected to a spin-valve type sensor element. In FIG. 6 of Pinarbasi there is shown the undercut bi-layer lift-off mask of photoresistive material deposited over PMGI that was schematically described above. Pinarbasi further discloses an ion-beam milling process followed by a deposition process, also as described above.
Han et al. (U.S. Pat. No. 6,007,731) discloses a lift-off mask that is used as an etch stencil for an ion-beam etch. The lift-off stencil is formed on an MR layer by depositing a blanket release layer and depositing over that a photoresist layer. The photoresist layer is then photoexposed and the release layer beneath it is undercut by use of an isotropic etchant.
Han et al. (U.S. Pat. No. 6,103,136) discloses the formation of an MR sensor in which there is used a lift-off stencil comprising a patterned release layer formed on the MR layer and a patterned photoresist layer formed so as to symmetrically overhang the release layer.
As recording densities on magnetic media continue to increase, the associated read head sensors must have correspondingly narrower track widths. Using the lift-off stencil formation associated with the method of the prior art to form read sensors with track widths less than 0.5 microns leads to several difficulties that impinge negatively on the efficiency and quality control of the manufacturing process. Basically, in manufacturing environments it becomes increasingly difficult to control the degree of undercut of the PMGI layer that lies beneath the photoresist layer. If, as a result, the undercut is too small, xe2x80x9cfencingxe2x80x9d (excessive buildup of conducting or magnetic material around the abutted junction which contacts the undercut layer) will occur at the edges of the abutted junction formed between the conductive lead layer and the GMR layer. This fencing can lead to shorting between the conductive lead layer and the upper shield of the read/write head that is formed above the conductive lead layer. A schematic depiction of such fencing is shown in FIG. 5a. 
On the other hand, if the undercut is too great, the overhang of the photoresist layer loses its support and can collapse during the layer deposition, as is illustrated schematically in FIG. 5b. If such collapse occurs, the removal of the lift-off stencil becomes a difficult problem and negatively affects the quality of the sensor as well as the efficiency of the manufacturing process. The present invention provides a new lift-off stencil structure that eliminates the problems of excessive undercut and inadequate undercut in a simple and novel manner.
A first object of this invention is to provide a method for forming conductive lead layers and magnetic bias layers on an abutted junction type GMR read element having a narrow track width.
A second object of this invention is to provide a method for forming conductive lead layers and magnetic bias layers on an abutted junction type GMR read element having a narrow track width, wherein the lead and bias layers so formed do not exhibit fencing (excessive material buildup in the vicinity of the junction which contacts the liftoff stencil) which can lead to subsequent electrical shorting between the conducting layer and the upper shield formed over the GMR sensor element.
A third object of this invention is to provide a method for forming conductive lead layers and magnetic bias layers on an abutted junction type GMR read element having a narrow track width, wherein removal of the lift-off stencil used in forming said junction is not rendered difficult by its collapse during said formation process.
A fourth object of this invention is to provide a method for forming conductive lead layers and magnetic bias layers on an abutted junction type GMR read element having a narrow track width, wherein said method is robust and easily implemented within a manufacturing environment.
In accord with the objects of this invention there is provided a bi-layer lift-off stencil wherein the upper level region which defines the active area of the GMR sensor has a novel suspension-bridge shape with no material from the lower layer beneath it and is not, therefore, subject to problems of inadequate or excessive undercutting of its lower layer relative to its upper layer.